Vertical roller mills, especially those common for grinding of cement raw materials, typically employ a hydraulic-pneumatic system to apply a grinding force to the material bed. During operation, these systems will contain pressurized hydraulic fluid in an isolated branch of the circuit consisting principally of cylinders and accumulators. This trapped pressure, along with the cylinder and accumulators, creates a hydraulic “spring”. The hydraulic spring serves two purposes. First, it provides the grinding force to the rollers for the purpose of comminution. Second, it acts as a suspension system so the grinding rollers can accommodate changes in material depth and strength.
Typical vertical roller mill geometry has the rod side of the cylinder pressurized to create the grinding force. Various possibilities exist for the piston side. Some systems have non-pressurized oil which freely flows between the cylinder and tank. Other systems have means to evacuate this area, and operate with a partial vacuum. A third type, relevant to this invention, employs pressurized oil on the piston side. These counter-pressure hydraulic systems for vertical roller mills are well known in the cement industry. Pressurization of the piston side, at a much lower level than on the rod side, has been demonstrated to improve operational stability of vertical mills grinding cement raw materials.
During normal grinding, it is desirable to have a relatively flat force-displacement curve, i.e., a soft hydraulic spring. This softness, or low spring stiffness, contributes to maintaining a low mill vibration level. However, to prevent potentially damaging mill vibration or tire-to-table contact, the grinding force should be reduced or even removed completely if the material bed becomes unstable. This cushioning effect (that is, a decrease in grinding force at low bed depths) is one of the major benefits of counter pressure systems.
In traditional counter pressure systems, the cushion effect comes at the expense of increasing system stiffness. FIG. 1 illustrates force displacement curves A–D in such traditional counter pressure systems utilized in a roller mill. Since the cushion effect is directly proportional to the counter pressure magnitude, as the cushion effect is increased, that is, as one goes from the system depicted in curve A toward the system depicted in curve D, the system stiffness, or steepness of the force displacement curve, is also increased. It is one object of the invention, therefore, to eliminate the need to make trade offs between system stiffness and cushion effect.